Nerve terminal calcium controls synaptic strength in several ways. Brief but intense calcium increases near release sites trigger vesicle fusion, and small sustained increases in steady state calcium levels throughout nerve terminals can enhance synaptic strength. This picture is based on peripheral synapses and model systems; there is little direct information on central synapses. I propose to study these dual roles of calcium in governing synaptic transmission in the central nervous system. Previous studies of synaptic transmission in the mammalian brain have been limited in scope because most presynaptic terminals are small and inaccessible. Consequently there has been little direct information about presynaptic action of calcium in controlling the strength of these synapses. To overcome this barrier I have developed a technique to measure nerve terminal calcium from the tiny boutons in the central nervous system using calcium-sensitive fluorophores. Combining this method to monitor presynaptic calcium dynamics with whole cell recording techniques to monitor post synaptic currents, the participation of calcium in synaptic transmission will be directly examined at two excitatory synapses in brain slices from rat and guinea pig; the mossy fiber synapse between dentate granule cells and CA3 pyramidal cells in the hippocampus, and the parallel fiber to Purkinje cell synapse in the cerebellum. The exceptionally large mossy fiber terminals offer technical advantages when measuring nerve terminal calcium, while the small parallel fiber terminals are more typical of others in the CNS. The approach taken should be applicable to a broad range of synapses in the brain. We will first measure calcium transients in nerve terminals and determine the calcium sources. The ability to measure the calcium entering a nerve terminal in response to an action potential will allow us to determine the relationship between calcium influx and release, and to examine the role of different types of calcium channels in triggering neurotransmitter release. Measurements of steady-state calcium levels will allow us to examine the role of small activity-induced increases in calcium in enhancing synaptic strength. Ultimately we will examine the contribution of changes in calcium influx and alterations of calcium dynamics to the modulation of synaptic strength by chemical messengers. These studies will lead to a deeper understanding of the factors controlling the strength of central synapses, and promise to be important in understanding synaptic plasticity, short and long term memory, as well as a number of complex conditions including epilepsy, schizophrenia and depression.